In the electric discharge lamp art it is well known that the performance of some discharge lamps can be enhanced by operating them at or near optimal temperatures. Accordingly, many lamps are provided with temperature modifiers of various types to hold them near the optimal operating temperatures. For example, fluid may be circulated in a casing surrounding a lamp envelope to cool the lamp. Alternatively, electric heaters or heat insulating jackets may be provided to operate the lamp at higher temperatures.
One type of discharge lamp that gives brighter light output at optimal temperatures is the low-pressure mercury discharge lamp. These lamps are widely used as light sources for optical instruments because of their availability and because of their bright 254 nm ultraviolet line. Optimizing the lamp temperature not only maximizes the intensity of the lamp output, but also makes the lamp output less sensitive to variations in temperature. Stable light output is a lamp characteristic which is important in many optical instrument applications. Each lamp design has a different optimal temperature, but it is not clear, a priori, what that temperature will be. Empirically, it is discovered that lamp output intensity is at a maximum when the lamp temperature is held near 40.degree. C. for ordinary fluorescent room lighting lamps, and 60.degree. C. for other lamps. The optimal temperature also depends in part on which spectral line or lines the optical instrument is to use.
In U.S. Pat. No. 4,074,163, Van der Leeuw discloses a gas-and-vapor discharge lamp having a discharge tube which is provided with a heat shield. The heat shield is connected to a bimetal element of the lamp such that, when the temperature of the discharge tube is raised or lowered, the shield is moved, respectively, further from or towards the discharge tube. Accordingly, the lamp reaches its optimal operating temperature rapidly after starting and temperature fluctuations during operation are small.
One problem associated with metal-vapor-type discharge lamps, such as the low pressure mercury discharge lamp, is condensation of the metal on the envelope walls. It is common practice to provide mercury lamps with an excess of mercury liquid so that mercury will be available to make up any losses incurred through such processes as burial of mercury ions in the region or end near the cathode. Mercury's vapor pressure decreases approximately exponentially with a decrease in operating temperature. At 125.degree. C., the vapor pressure of mercury is approximately one torr; at 100.degree. C., 0.273 torr; at 60.degree. C., 0.025 torr. If the temperature at any point on the wall of the tube decreases to below the point where the partial pressure of mercury equals the vapor pressure, condensation will occur at that point. In general, the vapor pressure everywhere in the tube will come to equilibrium with the coldest point on the tube envelope, and droplets of mercury will tend to collect at that coldest point on the envelope.
Condensation tends to obstruct light output, making the output intensity unstable. Condensation is particularly a problem for end-viewed lamps which are viewed through a window at an end of the lamp. Droplets of mercury may tend to condense on this window, making the output unstable. It is therefore of great importance to provide mercury discharge lamps which insure that mercury condensation will not harm the functioning of the lamp. One possible solution is to decrease the partial fill pressure of mercury for lamps intended to run at cooler temperatures. However, this limits the potential life of the lamp.
It is an object of the present invention to provide a vapor discharge lamp with support structure which operates at or near its optimum temperature without light-output-obstructing condensation.
It is another object of the present invention to provide a vapor discharge lamp having a temperature modifying structure such that the lamp operates at near optimum temperature and has substantially stable light output.